Garry Tan was in his happy place. Surrounded by food trucks and techies basking in San Francisco’s September sun, the CEO of Y Combinator snapped selfies with entrepreneurs as he meandered through a crowd of 2,700 attendees at the startup accelerator’s annual alumni event.

A week earlier, though, Tan had been blowing a fuse. In a series of barbed tweets, Tan had blasted Ali Partovi—the leader of a rival accelerator, Neo—accusing him of slandering Y Combinator, while another Y Combinator partner, Michael Seibel, described Partovi as a dishonest bully and said he’d heard “so, so much dirt” about him. The impetus for the mudslinging: Earlier in the day, at a hackathon on the Massachusetts Institute of Technology’s campus, Partovi told college students that Neo offers a better ratio of mentors to startups than Y Combinator does.

As trash-talking goes, Partovi’s comments were relatively bland, which prompted confusion among some startup founders about why Y Combinator’s leaders went after him with such vehemence. After all, in the world of accelerators, Neo is a mere minnow and Y Combinator is the whale that helped launch Stripe, Airbnb and a pantheon of other blockbuster startups. Could it be that Y Combinator was beginning to feel a bit less secure about the unshakability of its position in the startup ecosystem?